Pumice is a porous, volcanic rock used as a component of bonsai soil. Pumice varies in color and grade. Most have some ability to absorb water and yet retain a pore space that promotes good root growth. It is one of the few US native product suitable for replacement for Akadama, due to the similar hydration and ion exchange properties, however it is to hard to allow for direct root penetration.
Like lava, pumice is a product of volcanic eruption, but there the similarity ends. Rather than solidified molten rock flow, pumice is the non-crystalline solid form of molten rock that is blasted out of a volcano at extremely high pressure and temperature. It is this rapid decompression that happens as it leaves the volcano that gives pumice its softer texture and more interconnected pore structure.
With a cec that hovers around 75 meq/100g (depending on source) it requires less frequent fertilizing that most other soil components when used alone. Pumice can hold large quantities of water - up to four times its own weight. This, coupled with the soft and easily powdered surface means that when used alone or with other absorbent materials, careful watering is necessary to avoid water logging.
Although soft, roots are unable to penetrate pumice - even the surface pores since they are far too small. However, since the surface of each particle is soft, roots do seem to enjoy their company very much, and ramify well. After a while, the roots seem to break down the surfaces of the particles to form a sort of micro environment of small particles and dust that keeps them snug and fine.
Pumice drains well but tends to drain less well after a couple of years in use. However, drainage is never impeded to the extent that it becomes a problem.